Vertical distribution and source analysis of metals in Bohai Bay based on sedimentary cores

Bohai Bay, a representative semi-enclosed gulf in northern China, exhibits complex metal sources due to coupled effects of intensive anthropogenic activities and natural sedimentation processes. To clarify the regulation of nearshore metals by major water conservancy projects in the 1970s (e.g. sluice-gate and dam constructions), we examined the particle size, major metal elements (Al、Ca、Fe、K、Mg、Mn、Na) and heavy metal (Cr、Cu、Zn、Cd、Pb、Sb、Hg) contents of the nearshore sedimentary cores in the western Bohai Bay. By integrating the principal component analysis (PCA) with data normalization methods, we revealed the long-term impact of the Haihe River sluice construction projects in the 1970s on the metal sources and sedimentary patterns. Results indicated that the sedimentary cores in Bohai Bay were predominantly composed of silt and clay with particle size (Ф value) ranging from 4.83 to 6.69, and the sediment transport exhibited a saltation-dominated mode. There were significant peak fluctuations in the vertical distribution of the metals in the sedimentary cores. Factor analysis and normalization analysis indicated that the distribution pattern of metals in the surface sediments near the coastal zone of estuaries flowing into the sea before the 1970s was jointly influenced by the natural transport of sedimentary cores and the resuspension-deposition process in coastal areas and tidal sections of rivers flowing into the sea (cumulative variance contribution>70%). However, with the completion of the flood-control gates of the rivers flowing into the sea in Haihe River Basin one after another, since the early 1970s, the metal distribution patterns in the near-shore waters in the estuary had been dominated by natural geochemical processes, and only some local areas near the industrial zone had shown the characteristics of anthropogenic pollution. Therefore, the metal migration in the surface sediments in most areas of Bohai Bay was mainly controlled by the resuspension-deposition process of the sedimentary cores within the bay. In some coastal industrial areas, anthropogenic emissions and other factors remained the main sources of major metals. The research has revealed the dual impact of natural and human activities on the sedimentary environment, which has had a long-term effect on the temporal and spatial differences of metals in the sediments of Bohai Bay. These findings provide a scientific basis for the environmental management of the coastal zone.